The invention relates to novel manganese dioxide electrodes comprising modified, electrochemically active manganese dioxide, a method for producing said novel manganese dioxide electrodes, and also their use in primary electrochemical cells.
Typical constituents of an alkaline primary cell are a cathode consisting of manganese dioxide, an anode, preferably of zinc, an alkaline electrolyte and an electrolyte-permeable separator material.
The zinc electrode consists, as a rule, of large-area zinc powder and a gelling agent, for example carboxymethylcellulose, as stabilizer. Also known, however, are zinc powder electrodes pressed or sintered cold or hot with or without binder. In addition to amalgamated zinc anodes, mercury-free zinc anodes are being used to an increased extent.
The alkaline electrolyte generally consists of an aqueous potassium hydroxide solution. It may, however, also be solutions of other hydroxides, such as sodium hydroxide or lithium hydroxide solutions and also their mixtures.
The separator material situated between the electrodes has the purpose of electronically isolating the two electrodes.
An electrolyte comprising pyrolusite, a manganese dioxide having xcex3-structure, which has a very high electrochemical activity is frequently used as cathode material. To increase the electrical conductivity, carbon particles, soot particles or graphite particles are usually added to such manganese dioxide electrodes. Organic or inorganic additives are used as binders.
U.S. Pat. No. 5,342,712 describes cell discharge times prolonged by 5 to 15% at high and at medium discharge currents as a result of adding titanium dioxide having anatase structure to the active mass of the manganese dioxide cathode. At the same time, such cells have a cell voltage which is increased by about 60 mV during the discharge. At low discharge currents, however, a negative effect is exhibited. The mode of operation of the titanium dioxide of this structure is explained by an increased ion mobility in this material during the discharge and a reduction in the polarization associated therewith, which results in turn in a longer discharge time. This effect is not achieved, according to this publication, by adding titanium oxide having rutile structure.
U.S. Pat. No. 5,532,085 describes the addition of CaWO4, MgTiO3, BaTiO3, CaTiO3, ZnMn2O4 and Bi12TiO20 and combinations of these oxides to the manganese dioxide cathode. Under various discharge conditions, up to 10% longer discharge times were measured on primary cells as a result of these additives.
These known processes for prolonging the limited discharge time of primary electrochemical cells by adding titanium dioxide have, however, substantial disadvantages for large-scale industrial use. As already stated in connection with U.S. Pat. No. 5,342,712, good cell characteristics can be achieved by adding anatase TiO2 only for high and medium discharge currents. At low discharge rates, this effect cannot be detected. Furthermore, the specified improvements can be achieved only by using high-purity titanium dioxide particles. The prolonged discharge times described in U.S. Pat. No. 5,532,085 are not clearly comprehensible.
The object of the present invention was therefore to provide manganese dioxide electrodes whose use in galvanic cells, electrochemical cells, in particular in primary cells result in products having improved properties, in particular having prolonged discharge times and increased cell voltages during the discharge, and to be specific, both at high and at low discharge rates. It was also the object to provide an inexpensive, readily performed process for producing these modified manganese dioxide electrodes.
The object is achieved by manganese dioxide electrodes which comprise inorganic particles. These particles may be either plain or coated inorganic particles. Suitable as plain inorganic particles are, in particular, mica particles. Surprisingly, it was found by experiments that by blending the manganese dioxide normally used as cathode material with commercially obtainable mica particles, a starting material is obtained for producing manganese dioxide electrodes from which cathodes can be produced which have considerably improved properties.
The present invention therefore relates also to a manganese dioxide electrode which contains mica.
In this connection, the generally known and commercially obtainable mica types can be used. In a particularly preferred embodiment of this invention, the mica is used in ground form, mica particles having certain particle sizes being present. Preferably, mica particles having a particle size of 5-50 xcexcm are used. Obtainable commercially, for example, are various micas of certain particle sizes, such as, for example, the F-mica (Merck KGaA, Darmstadt) having a particle size of 5-25 xcexcm, the M-mica (Merck KGaA, Darmstadt) having a particle size of  less than 15 xcexcm or also the N-mica (Merck KGaA, Darmstadt) having a particle size of 10-50 xcexcm. It goes without saying, however, that micas can also be used which have not been previously subjected to a specific treatment.
The coated particles can be those whose support particles consist of a material selected from the group consisting of mica, SiO2, Al2O3, ZrO2 and ZnO. Single or multiple coatings of said particles can be built up of dielectric and, in particular, of ferroelectric, piezoelectric or pyroelectric substances. Such coatings may consist of titanates, stannates, tungstates, niobates or zirconates; in addition, however, silicate coatings are also possible, depending on the type of basic particle selected. Particles having coatings consisting of mixtures of said substances are also suitable. Suitable inorganic particles may also have coatings consisting of metal oxides from the group consisting of Fe2O3, NiO, CoO, ZrO2, SnO2, TiO2, Sb2O3, PbO, Pb3O4 or Bi2O3 and mixtures of the latter. The single coatings consisting per se of one substance may be doped with foreign ions, such as, for example, SnO2 coatings doped with foreign ions.
The manganese dioxide used as basic material can be present in a structure comprising water of crystallization.
The abovementioned object is achieved, in particular, by manganese dioxide electrodes which comprise coated inorganic particles in an amount of 0.01 to 20% by weight, relative to the amount of manganese dioxide comprised in the electrode.
The manganese dioxide electrodes are produced by
a) homogenizing the manganese dioxide powder with an inorganic powder consisting of plain or singly or multiply coated inorganic particles,
b) optionally blending the mixture with an organic or inorganic binder and a conductive additive (preferably graphite), and
c) making the product obtained into an electrode.
The invention likewise relates to this production process.
Manganese dioxide electrodes according to the invention can be used to produce galvanic cells, electrochemical cells, primary batteries and in the latter case, 8 button-cell batteries, in particular.
Surprisingly, it was found by experiments that, as a result of blending the manganese dioxide conventionally used as cathode material with inorganic coated particles, so-called pearlescent pigments, obtainable commercially, a starting material is obtained for the production of manganese dioxide electrodes from which cathodes can be produced which have substantially improved properties. These pigments are inorganic particles which may be coated with a very wide variety of substances.
The experiments performed have shown that cathodes having prolonged discharge times are obtained by adding plain inorganic particles, such as, for example, mica, or inorganic coated particles if such inorganic particles are added in the form of mica or of coated mica particles, SiO2 particles, Al2O3 particles, ZrO2 particles or coated ZnO particles to the manganese dioxide in an amount of 0.01 to 20% by weight, relative to the amount of manganese dioxide. The amount added in each case depends on the intended use of the manganese dioxide electrodes produced. Whereas the addition of even minor amounts of about 0.01% by weight of the abovementioned particles exerts an appreciable effect on the discharge times of commercial batteries, additions of up to 20% by weight to cathode materials of button-cell batteries may be expedient.
The modification of the cathode material with plain inorganic particles, such as mica, achieves a marked increase in the capacity of the electrochemical cell compared with commercially available zinc/manganese oxide batteries whose cathodes are unmodified.
In particular, the modification with inorganic coated particles achieves an increase in the capacity of the electrochemical cell of 10 to 30% compared with commercially available zinc/manganese oxide batteries whose cathodes are unmodified. An increase in capacity of 30% can be achieved, in particular, by adding 20% by weight of inorganic coated particles to the manganese dioxide used.
Accordingly, it may be expedient to vary the amount of particles added depending on the type of particles and the use of the electrodes.
Plain inorganic particles which have proved useful for modifying the manganese dioxide used for producing electrodes are commercially available mica particles, which have already been described above.
Commercially available, coated inorganic particles comprising mica as support material have proved particularly suitable for modifying the manganese dioxide used to produce electrodes. Such materials are:
mica coated with titanium dioxide in anatase or rutile modification
mica coated with SiO2 and/or SnO2 and/or TiO2,
mica coated with alkaline-earth titanates, alkali-metal titanates (Mg, Ca, Sr, Ba titanates) and/or lead titanate
mica coated with stannates, tungstates, niobates or zirconates
mica coated with metal oxides (Fe2O3, NiO, CoO, ZrO2, SnO2, Sb2O3, PbO, Pb3O4 or Bi2O3)
mica coated with ZrO2 
mica coated with mixtures of said oxides and titanates.
Also suitable, however, for the modification are those inorganic particles which are coated in the same way and in which SiO2 particles, Al2O3 particles, ZrO2 particles serve as support material. Good effects are achieved with the aid of particulate materials whose support materials can already be polarized per se, which is not, however, a requirement since improved capacities are also measured with materials whose support particles do not have these properties. It has proved particularly advantageous, however, if the coatings consist of dielectric substances, in particular of ferroelectric, piezoelectric or pyroelectric substances, such as, for example, titanates, stannates, zirconates, tungstates, niobates or others.
Surprisingly, it was found in experiments that the use of particles according to the invention having titanium dioxide coatings results, in contrast to that in U.S. Pat. No. 5,342,712, in an appreciable increase in the capacity of the experimental cell, regardless of whether the coating has an anatase structure or a rutile structure. It has also been found that the advantageous result is achieved without high-purity starting substances being used for the modification. Equally good results are achieved if particles whose surfaces are coated with metal oxides from the group consisting of Fe2O3, NiO, CoO, ZrO2, SnO2, TiO2, Sb2O3, PbO, Pb3O4, Bi2O3, WO3, NbO or with mixtures of said metal oxides are used to modify the electrode material. Surprisingly good capacity increases are achieved by adding particles whose surface coatings are doped with foreign ions, such as, for example, SnO2 coatings doped with antimony.
To produce the actual cathode material, the manganese dioxide powder is blended with the desired amount of particulate powder and homogenized in a manner known to the person skilled in the art. The homogenization can be performed by grinding in ball mills or attrition mills. In the experiments performed, grinding with ball mills for about eight hours and longer has proved beneficial. The product homogenized in this way can then be blended with further additives, such as, for example, with organic or inorganic binders and conductivity additives. PTFE, latex and other binders known to the person skilled in the art for this purpose can be added as organic binders. Portland cement may serve as inorganic binder. Particularly suitable is PTFE. Suitable conductivity additives are soot, graphite, steel wool and other conductive fibres. Particularly good results were achieved by adding soot and graphite in an amount of 4-10, in particular of about 5% by weight, relative to the total amount.
The powder blended with all the additives is then made into electrodes in a manner known per se. This can be done by pressing at very high pressure between wire fabrics consisting of an inert material, such as, for example, nickel. Optionally, this can be followed by a treatment at elevated temperature, a so-called annealing.
Electrodes produced in this way can be used in a known manner to produce primary galvanic cells in which a zinc electrode conventionally serves as counter-electrode in the presence of an alkaline electrolyte. Other designs of suitable galvanic cells are, however, also possible. Thus, the viscosity of the per se aqueous electrolyte can be increased by various additives, such as, for example, gelling agents, silica gel or others. A suitable polymer fabric or polymer nonwoven material can be provided as separating material between the electrodes and a spacer can be inserted if this should be necessary. Materials consisting of PVA, polypropylene or other inert polymers may serve as polymer nonwoven materials. Spacers such as those known from commercially obtainable batteries can have a corrugated form or consist, for example, of PVC.
For experimental purposes, electrodes were produced from the manganese dioxide mixtures according to the invention by adding, after the grinding, a conductivity additive and a binder in each case. The mixture thus obtained was pressed between two nickel-wire gauzes to form cathodes.